PURPOSE: To obtain a numeric value of errors in cause quantitatively by using a measuring data of the radius of rotation of circular motion measured at each specified angle of rotation as dependent variable to specify a cause resulting in errors in the circular motion of a machine.

CONSTITUTION: A sum and (sum of products) computing section 7 inputs two independent variables stored in a memory section 3 according to selection and setting by a parameter setting section 5. A measuring data stored in the memory section 3 is taken in and is used as dependent variable to perform a calculation for the sum or self and mutual sum of products of the in dependent variables and the depending variables. A mean/coefficient computing section 9 calculates a mean and a coefficient of a simultaneous equation from the sum and the sum of products of the independent variables and the dependent variables. A regression coefficient/constant computing section 11 solves the simultaneous equations to calculate a regression coefficient of each independent variable while a constant of the regression and a contribution rate of the regression are calculated from the regression coefficient and the mean. A value of regressing and an error component are calculated by multiple regression formula based on the regression coefficient, the constant of regression and the independent variables.